As is known, since the early days of the telephone and telegraph, communications signals have traditionally been transmitted over copper wires and cables. In recent years, however, an increasing volume of communications signals are transmitted in the form of light beams over optical waveguides. Various types, of peripheral equipments, such as connectors and switches based on optical waveguides have been developed. In particular, a technology known as integrated optics is widely used in handling optical communications signals. Using this technology, communications signals in the form of light beams are transmitted through optical waveguides formed in substrates made of electro-optical materials such as lithium niobate (LiNbO3), which is probably the most widely used material due to its enhanced electro-optical properties and to the possibility of making low-loss optical waveguides.
Although integrated optics is now widely used in transmitting signals, fulfillment of continuous demand for optical devices operating at higher and higher frequencies is limited by the difficulty of making optical frequency shifters with appropriate characteristics.
FIG. 1 shows a standard representation of an optical frequency shifter having an optical input, where an input optical signal with an input optical frequency is received, an electrical input, where an RF electrical drive signal with an electrical frequency in the microwave range is received to electrically drive the optical frequency shifter, and an optical output, where an output optical signal is supplied with an output optical frequency equal to the input optical frequency of the input optical signal increased of the electrical frequency of the electrical drive signal. FIG. 1 also shows the optical spectra of the input and output optical signals, as well as the time pattern of the electrical drive signal.
FIG. 2 show a schematic representation of a known optical frequency shifter 1, which basically includes an optical waveguide structure 2 formed in a substrate 3 (shown in the following FIG. 3) of, an electro-optical material, normally lithium niobate (LiNbO3), in a conventional manner, for example by selectively diffusing titanium into the substrate 3.
The substrate 3 has an X-cut crystalline structure, i.e. a crystalline structure with an X crystal axis which is orthogonal to a main surface 3a of the substrate 3 (i.e. the surface of largest area); the orientation of the crystalline structure causes electrical and optical fields generated by the electrical drive signal and the input optical signal to couple mainly along a Z crystal axis of the crystalline structure, i.e. the electro-optical coupling along the other two crystal axes is negligible compared to the electro-optical coupling along the Z crystal axis.
In fact, the electro-optical effect causes the refractive index of the electro-optical material to change spatially as a function of intensity and direction of an external electrical field applied thereto. In particular, along a given spatial direction the refractive index changes proportionally to the intensity of the electrical field along that direction. The refractive index changes along the three crystal axes X, Y and Z of the electro-optical material may be computed by multiplying scalarly the electrical field vector by a 3×3 matrix of electro-optical coefficients. In the case of a LiNbO3 crystal, among the 3×3 electro-optical coefficient matrix the electro-optical coefficient having the highest value is r33(≈30 pm/V), which relates the refractive index change experienced by electromagnetic waves polarized along the Z crystal axis to the component of the electrical field along the same axis.
The optical waveguide structure 2 comprises a Y-shaped waveguide structure 4 and a reversed Y-shaped waveguide structure 5 coupled in series. The Y-shaped waveguide structure 4 includes an input branch 6 configured to be coupled, in use, to an input optical fiber (not shown), and a pair of mutually optically coupled branches 7 branching off from the input branch 6. The reversed Y-shaped waveguide structure 5 comprises a pair of mutually optically uncoupled branches 8 coupled to respective mutually optically coupled branches 7 of the Y-shaped waveguide structure 4, and merging into an output branch 9 configured to be coupled, in use, to an output optical fiber (not shown). One of the mutually optically uncoupled branches 8 of the reversed-Y-shaped waveguide structure 5 is so structured to induce a phase change of π radians in the optical signal propagating therealong.
In particular, the two mutually optically coupled branches 7 of the Y-shaped waveguide structure 4 are spaced apart by a distance S (first inter-waveguide spacing) short enough to ensure mutual optical coupling, typically ranging from 5 to 10 μm, while the two mutually optically uncoupled branches 8 of the reversed-Y-shaped waveguide structure 5 are spaced apart of a distance D (second inter-waveguide spacing) high enough to prevent mutual optical coupling. Additionally, the coupling degree κCOUP of the mutually optically coupled branches 7 of the Y-shaped waveguide structure 4 is a function of the first inter-waveguide spacing S via a proportionality factor e−αS, where α is the inverse of the distance at which the coupling factor reduces to a fraction 1/e of the extrapolated value at zero distance.
The optical frequency shifter 1 further comprises 1-30 μm-thick electrically conductive electrode structure 10 formed of gold or similar metals above the main surface 3a of the substrate 3, in a conventional manner, over the mutually optically coupled branches 7 of the Y-shaped waveguide structure 4.
In particular, as shown in FIG. 3, the electrode structure 10 includes an inner electrode 11 arranged between the mutually optically coupled branches 7 of the Y-shaped waveguide structure 4, and a pair of outer electrodes 12 arranged outside the mutually optically coupled branches 7, on opposite sides of, and symmetrically to the inner electrode 11. A dielectric (e.g. SiO2) buffer layer 13 is arranged between the main surface 3a of the substrate 3 and the electrode structure 10 to prevent or minimize absorption of optical power by the electrode structure 10.
The outer electrodes 12 are generally grounded, while the inner electrode 11 is supplied with an electrical drive signal, having an electrical frequency Ω and a momentum βΩ, which results in an RF drive voltage applied between the inner electrode 11 and the outer electrodes 12. The RF drive voltage generates opposite electrical fields between the inner electrode 11 and the outer electrodes 12; the electrical fields have a direction substantially parallel to the main surface 3a (and to the Z crystal axis) and traverse a respective one of the mutually optically coupled branches 7, with opposite orientations. As the only electro-optical coefficient with a non-negligible value is r33, these opposite electrical fields induce opposite refractive index changes in the mutually optically coupled branches 7 of the Y-shaped waveguide structure 4.
An input optical signal, consisting of a single symmetric mode with propagation index NS, optical frequency ωS and momentum βS, and received at the input branch 6 of the optical frequency shifter 1, propagates along the Y-shaped waveguide structure 4, which, due to the mutual optical coupling of its branches, is operatively seen by the input optical signal as a single waveguide which may support two distinct supermodes with opposite parity known as symmetric supermode and antisymmetric supermode. In the absence of other phenomena, i.e no intrinsic asymmetries and/or electrical perturbations only the symmetric supermode would propagate without exciting the antisymmetric supermode. Therefore, only a single symmetric supermode with the same power as that of the input optical signal, propagation index NS, optical frequency ωs and momentum βS starts propagating along the Y-shaped waveguide structure 4.
During the propagation, the symmetric supermode experiences the afore-mentioned opposite refractive index changes, and this results in the energy of the symmetric supermode transferring partially to the antisymmetric supermode, which starts propagating in addition to the symmetric supermode. The antisymmetric supermode has a frequency ωA equal to the frequency ωS of the symmetric supermode up-shifted of the frequency of the electrical drive signal applied to the inner electrode 11 (ωA=ωS+Ω), a momentum βA equal to the momentum βS of the symmetric supermode up-shifted of the momentum βΩ of the electrical drives signal applied to the inner electrode 11 (βA=βS+βΩ) (due to a momentum conservation constraint), and a propagation index NA.
Therefore, at the output of the Y-shaped waveguide structure 4 both the residual symmetric supermode and the antisymmetric supermode are present, which enter the reversed Y-shaped waveguide structure 5, in each branch of which a composed optical signal propagates, having half of the power of the input optical signal and comprising both a symmetric mode with two lobes with equal sign, and an antisymmetric mode with two lobes of opposite signs. During the propagation along the reversed Y-shaped waveguide structure 5, a phase change of m radians is induced in one of the two composed optical signals, so obtaining in the output branch 9 of the reversed Y-shaped waveguide structure 5, where the two composed optical signals merge, the summation of the two lobes of the antisymmetric mode in a single output optical signal having a frequency ωA, and the cancellation of the two lobes of the residual symmetric mode.
A resonance frequency Ω of the optical frequency shifter 1, i.e. the electrical frequency Ω of the electrical drive signal which, when supplied to the electrical input of the optical frequency shifter, produces the maximum shift efficiency, is imposed by the design specifications of the optical frequency shifter itself, and in particular it is directly proportional to the coupling degree κCOUP of the mutually optically coupled branches 7 of the Y-shaped waveguide structure 4; the coupling degree κCOUP is, in turn, proportional to e−αS, where S is the first inter-waveguide spacing of the mutually optically coupled branches 7 as it is given by the following formulas:
      Ω    =                  c        ⁡                  (                                    N              S                        -                          N              A                                )                            λ        ⁡                  (                                    n              RF                        +                          N              A                                )                                                  N          S                -                  N          A                    ∝              κ        COUP            ∝              ⅇ                              -            α                    ⁢                                          ⁢          S                      ,  Thus an increase of operating frequency may be obtained by a reduction of the first inter-waveguide spacing S of the mutually optically coupled branches 7.